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Building the Survivor: TX/RX Switching

This is probably one of the easiest parts of the circuit to build–only two IC’s and two other components. It’s a clever design, in my opinion. Let’s get to it.

The purpose of this part of the circuit is to switch the rig between transmit and receive. I pointed out in the previous installment that the VFO and BFO get switched between the two mixers U1 and U2 depending on whether the rig is transmitting or receiving. All of the things that need to be switched when changing from transmit to receive are controlled by U5, the Tiny 13A microcontroller. In this section we’re going to worry only about switching the two oscillators.

The outputs of both oscillators are connected to U4, the 74HC4053 analog switch IC, and U5 is connected to U4 to “throw the switch,” so to speak. Here’s a pinout diagram for U5 (I swiped it from a data sheet, which you should be able to readily find for yourself on the web):

Pinout for the 74HC4503 analog switch.

Pins 9, 10, and 11 (labeled A, B, and C) are used to switch inputs and outputs. For example, when pin 11 (A) is grounded, pin 14 is connected to pin 12 (ax), but if A is taken high (connected to 5V, that is), pin 14 gets connected to pin 13 (ay). Similarly, if pin 10 (B) is grounded, pin 15 is connected to pin 2 (bx), but if B is taken high, pin 15 is connected to pin 1 (by). Pin 9 (C) works the same way with pin 4 and pins 5 (cx) and 3 (cy).

Knowing that, take a look at the schematic and note that pin 4 is connected to U1 and pin 15 is connected to U2 (the B and C outputs). The VFO output is connected to pin 1 (by) and pin 5 (cx), and the BFO output is connected to pin 2 (bx) and pin 3 (cy). Pin 2 on U5 (the microcontroller) is tied to both pins 9 and 10 (the B and C switches). U5 is programmed so that when the rig is receiving, U5 pin 2 is high, and when transmitting, U5 pin 2 is low. if U5 pin 2 is high, then U4 pin 1 (by), the VFO, is connected to U2 and U4 pin 3 (cy), the BFO, is connected U1. If U5 pin 2 is low, then U4 pin 2 (bx), the BFO, is connected to U2 and U4 pin 5 (cx), the VFO, is connected to U1. That make take a moment to sort out, but it’ll eventually make sense.

Okay, time to build. First, install the sockets for U4 and U5. Make sure you install them in the correct orientation so that the notch at one end of each socket corresponds to the notch on the board outline. Next, install R29 and C45. Finally, install the IC’s themselves, making sure that the little dimple in the corner of each IC is closest to the end of the socket with the notch. You can also install header pins for the PTT/MIC/GND pads where the microphone will eventually plug in–it’ll make it easier to short the PTT line to ground for testing. Here’s what the board looks like with the parts installed:

The Survivor board with the parts for TX/RX switching installed.Close-up of the TX/RX switching section.

To test this part of the circuit, you’ll once again need the digital dial. We’ll be measuring the frequency at pin 6 of both U1 and U2 while we switch the rig from receive to transmit by shorting PTT to ground. First, leave PTT disconnected from ground to stay in receive mode. You should see in this case that the frequency at U2 pin 6 is that of the VFO (~5 MHz) and the frequency at U1 pin 6 is that of the BFO (9 MHz). When you short PTT to ground, those frequencies will reverse–BFO frequency at U2 pin 6 and VFO frequency at U1 pin 6. Cool, huh?